Stars

Nature's Coolest "Stars"

Brown dwarfs serve as laboratories for understanding gas-giant extrasolar planets, as well as the faint end of the star formation process. The coldest known brown dwarfs have temperatures of ~400 - 700 K, emit a million times less power than does the Sun, possess molecule-dominated spectra (e.g. water, methane, and ammonia) and atmospheres that are more akin to Jupiter than any star.

Mike Liu has been using high angular resolution imaging from the Keck laser guide star adaptive optics (LGS AO) system to search for binaries among the field brown dwarf population.

This program has three major aspects:

Identifying even cooler objects, found as companions to the coolest known brown dwarfs. The most extreme discovery has been the CFBDSIR J1458+10AB system (shown here), which is the coldest known binary to date (500 K + 370 K binary).

Studying the cooling sequence of brown dwarfs, as they evolve from spectral types of late-M, to L, then T, and ultimately to Y dwarfs.

Astrometric monitoring of these binaries over 5 - 10 years. Such monitoring yields direct dynamical masses, which are the very best way to test current theoretical models of brown dwarf evolution and their ultracool atmospheres.

Metal-Poor Stars

The light elements, lithium, beryllium and boron, provide tracers for many aspects of astronomy including stellar structure, Galactic evolution, and cosmology.

Ann Boesgaard and three graduate students have been using the Keck Telescope to measure the abundance of Be in 117 metal-poor stars and compare it with the abundance to that of other elements such as iron and oxygen. They found that it is likely that the formation of Be in the accretive stars was primarily in the vicinity of type II supernovae, while the Be in the dissipative stars was primarily formed by Galactic cosmic-ray spallation.

Orphaned Protostars

Stars, and perhaps always, form in small multiple systems. This gives rise to strong dynamical interactions between the stellar embryos: a triple system usually evolves into a tight binary and a single object that is ejected, either into a distant orbit or into an escape.

Bo Reipurth has been performing numerical N-body simulations revealing that such break-ups of triple systems most frequently occur during the protostellar stage, when the stellar embryos are embedded in their placental cloud cores and are still growing. When an embryo is ejected before it has grown to a mass of 0.08 solar masses, it will forever remain a brown dwarf.

In many cases, protostellar objects are ejected with insufficient momentum to climb out of the potential well of the cloud core and associated binary. These loosely bound companions can travel out of their dense cloud cores to distances of many thousands of AU before falling back and eventually being ejected into escapes as the cloud cores gradually disappear and the gravitational bonds weaken. Such orphaned protostars offer an intriguing glimpse of newborn stars that are normally hidden from view. A number of such orphans have been identified in nearby star-forming regions in the vicinity of deeply embedded protostars, for the first time allowing detailed studies of protostars at near-infrared and even at optical wavelengths.

Cool Star Library

John Rayner has been constructing a 0.8 - 5 μm spectral library of 210 cool stars, observed at a resolving power of 2000 with the medium-resolution infrared spectro­graph, SpeX, at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea.

The stars have well-established MK spectral classifications and are mostly restricted to near-solar metallicities. The sample not only contains the F, G, K, and M spectral types with luminosity classes between I and V, but also includes some AGB, carbon, and S stars.

Potential uses of the library include studying the physics of cool stars, classifying and studying embedded young clusters and optically obscured regions of the Galaxy,and conducting evolutionary population syntheeis to study unresolved stellar populations in optically obscured regions of galaxies.

Spectra of M, S, and C giant stars of approximately the same effective temperature illustrating the effect of increasing carbon abundance during AGB evolution.